1. Field of the Invention
This invention relates to a polypropylene conjugate fiber having superior properties suitable to hot roll processing, as a raw material for a hot-melt adhesive polypropylene non-woven fabric. 2. Description of the Related Art
Hot-melt adhesive non-woven fabrics have been widely used in view of advantages of facilities, economy and hygienics due to the fact that no binder is required. Among the fabrics, since polyolefin non-woven fabrics are superior in the properties and economy, they have been used in many fields such as medical or hygienic materials such as for surgical gown, cover stock of diapers and sanitary napkins, etc., civil engineering materials, agricultural materials, industrial materials, etc., and as the raw materials for these materials, polypropylene fibers or polyethylene/polypropylene conjugate fibers have been used.
The process for producing hot-melt adhesive non-woven fabrics are classified into air-through process using hot air and hot roll process. The air-through process is suitably applied to polyethylene/polypropylene conjugate fibers. Non-woven fabrics according to the air-through process have a good tenacity and are soft, but as compared with the non-woven fabric according to hot roll process, have such drawbacks that the processing speed is so slow that the productivity is inferior and since polyethylene/polypropylene conjugate fibers are used, there is a waxy feeling specific of polyethylene.
Whereas, the hot roll process has such advantages that the processing speed is so rapid that it is advantageous in the aspect of productivity, and due to adhesion by way of hot press adhesion, even in the case of polypropylene fibers, it is possible to make up the fibers into non-woven fabrics.
Non-woven fabrics consisting of polypropylene fiber do not use polyethylene; hence the fabrics have an advantage of non-waxy feeling, but when the fabrics are produced according to hot roll process, the bonded points of the fibers become a film-form due to heat and pressure and fibers other than those at the bonded points are also compacted. As a result, there occurs a drawback that the hand of the obtained non-woven fabrics become inferior. If a low temperature or a low pressure is applied in order to prevent the hand from deterioration, the tenacity of the resulting non-woven fabrics is liable to be insufficient due to inferior adhesion. Thus, due to a small difference in the processing temperature, the tenacity becomes low or the hand becomes hard; hence conditions for producing a polypropylene non-woven fabric having a high tenacity and a soft feeling have been limited to very narrow ranges. Thus, development of a polypropylene fibers capable of obtaining a polypropylene non-woven fabric being soft and having a high tenacity, and having a broad width of processing temperature and suitable to hot roll process, has been long awaited.
As a polypropylene fiber suitable to hot roll process, Japanese patent application laid-open No. Sho 62-156310 discloses a polypropylene fiber consisting of an ethylene-propylene random copolymer having a softening point of 132.degree. C. or lower and containing a definite quantity of ethylene. However, this fiber has drawbacks that a non-woven fabric consisting of the fiber has a hard hand and the processing temperature range within which a non-woven fabric having a tenacity and a hand suitable to practical uses can be produced is very narrow.
Further, Japanese patent application laid-open No. Hei 2-112456 discloses a non-woven fabric consisting of low stereoregular polypropylene fiber having a specified isotactic pentad ratio. This non-woven fabric has a good hand but its tenacity is insufficient. Further, in general, such a fiber consisting of a low stereoregular polypropylene has a drawback that it has an inferior carding characteristic to raise a serious problem in the production of non-woven fabric.
Further, Japanese patent application laid-open No. Hei 2-264012 discloses a polypropylene fiber having a specified compound blended therewith, but its hand and tenacity are both insufficient.
As described above, a number of attempts of intending to provide a polypropylene non-woven fabric being superior both in the tenacity and hand have been carried out, but the properties of the produced non-woven fabric have not yet been sufficient and also the processing temperature range at the time of its production has been narrow; thus a satisfactory polypropylene fiber has not yet been developed.